Prior to v10.1, if requested by user, arch-capabilities is always on
despite the fact that CPUID advertises it to be off/unvailable.
This causes a migration issue for VMs that are run on a machine
without arch-capabilities and expect this feature to be present
on the destination host with QEMU 10.1.

Add a compatibility property to restore the legacy behavior for all
machines with version prior to 10.1.

Co-authored-by: Hector Cao <hector.cao@canonical.com>
Signed-off-by: Hector Cao <hector.cao@canonical.com>
Fixes: d3a24134e37 ("target/i386: do not expose ARCH_CAPABILITIES on AMD CPU", 2025-07-17)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.h     |  6 ++++++
 hw/i386/pc.c          |  1 +
 target/i386/cpu.c     | 17 +++++++++++++++++
 target/i386/kvm/kvm.c |  6 +-----
 4 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index e0be7a74068..414ca968e84 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -2314,6 +2314,12 @@ struct ArchCPU {
     /* Forcefully disable KVM PV features not exposed in guest CPUIDs */
     bool kvm_pv_enforce_cpuid;
 
+    /*
+     * Expose arch-capabilities unconditionally even on AMD models, for backwards
+     * compatibility with QEMU <10.1.
+     */
+    bool arch_cap_always_on;
+
     /* Number of physical address bits supported */
     uint32_t phys_bits;
 
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index bc048a6d137..d7f48150fdd 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -87,6 +87,7 @@ const size_t pc_compat_10_1_len = G_N_ELEMENTS(pc_compat_10_1);
 GlobalProperty pc_compat_10_0[] = {
     { TYPE_X86_CPU, "x-consistent-cache", "false" },
     { TYPE_X86_CPU, "x-vendor-cpuid-only-v2", "false" },
+    { TYPE_X86_CPU, "x-arch-cap-always-on", "true" },
 };
 const size_t pc_compat_10_0_len = G_N_ELEMENTS(pc_compat_10_0);
 
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 6d85149e6e1..fe369bb1284 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7539,6 +7539,20 @@ uint64_t x86_cpu_get_supported_feature_word(X86CPU *cpu, FeatureWord w)
 #endif
         break;
 
+    case FEAT_7_0_EDX:
+        /*
+         * Windows does not like ARCH_CAPABILITIES on AMD machines at all.
+         * Do not show the fake ARCH_CAPABILITIES MSR that KVM sets up,
+         * except if needed for migration.
+         *
+         * When arch_cap_always_on is removed, this tweak can move to
+         * kvm_arch_get_supported_cpuid.
+         */
+        if (cpu && IS_AMD_CPU(&cpu->env) && !cpu->arch_cap_always_on) {
+            unavail = CPUID_7_0_EDX_ARCH_CAPABILITIES;
+        }
+        break;
+
     default:
         break;
     }
@@ -10004,6 +10018,9 @@ static const Property x86_cpu_properties[] = {
                      true),
     DEFINE_PROP_BOOL("x-l1-cache-per-thread", X86CPU, l1_cache_per_core, true),
     DEFINE_PROP_BOOL("x-force-cpuid-0x1f", X86CPU, force_cpuid_0x1f, false),
+
+    DEFINE_PROP_BOOL("x-arch-cap-always-on", X86CPU,
+                     arch_cap_always_on, false),
 };
 
 #ifndef CONFIG_USER_ONLY
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 6a3a1c1ed8e..db40caa3412 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -503,12 +503,8 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
          * Linux v4.17-v4.20 incorrectly return ARCH_CAPABILITIES on SVM hosts.
          * We can detect the bug by checking if MSR_IA32_ARCH_CAPABILITIES is
          * returned by KVM_GET_MSR_INDEX_LIST.
-         *
-         * But also, because Windows does not like ARCH_CAPABILITIES on AMD
-         * mcahines at all, do not show the fake ARCH_CAPABILITIES MSR that
-         * KVM sets up.
          */
-        if (!has_msr_arch_capabs || !(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES)) {
+        if (!has_msr_arch_capabs) {
             ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
         }
     } else if (function == 7 && index == 1 && reg == R_EAX) {
-- 
2.51.0

On Tue, Sep 23, 2025 at 12:41:35PM +0200, Paolo Bonzini wrote:
> Date: Tue, 23 Sep 2025 12:41:35 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [RFT PATCH v2 1/2] target/i386: add compatibility property for
>  arch_capabilities
> X-Mailer: git-send-email 2.51.0
> 
> Prior to v10.1, if requested by user, arch-capabilities is always on
> despite the fact that CPUID advertises it to be off/unvailable.
> This causes a migration issue for VMs that are run on a machine
> without arch-capabilities and expect this feature to be present
> on the destination host with QEMU 10.1.
> 
> Add a compatibility property to restore the legacy behavior for all
> machines with version prior to 10.1.
> 
> Co-authored-by: Hector Cao <hector.cao@canonical.com>
> Signed-off-by: Hector Cao <hector.cao@canonical.com>
> Fixes: d3a24134e37 ("target/i386: do not expose ARCH_CAPABILITIES on AMD CPU", 2025-07-17)
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  target/i386/cpu.h     |  6 ++++++
>  hw/i386/pc.c          |  1 +
>  target/i386/cpu.c     | 17 +++++++++++++++++
>  target/i386/kvm/kvm.c |  6 +-----
>  4 files changed, 25 insertions(+), 5 deletions(-)

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>